1. Field of the Invention
The present invention relates to a hybrid vehicle, and more particularly to a parallel hybrid vehicle.
2. Description of the Related Art
Generally known parallel hybrid vehicles have an internal combustion engine as a primary propulsive power unit and an electric motor for generating an assistive output power for assisting the output power produced by the engine. The output powers (mechanical powers) generated by the engine and the electric motor are transmitted through a transmission to drive wheels of the hybrid vehicle.
For accelerating the hybrid vehicle, the electric motor is controlled to generate an assistive output power, and both the output power produced by the engine and the assistive output power from the electric motor are transmitted to the drive wheels. Therefore, the power requirements for accelerating the hybrid vehicle can be met, and the output power produced by the engine may be relatively small, thus reducing fuel consumption by the engine and exhaust gases emitted from the engine.
While the hybrid vehicle is decelerating, the electric motor is operated as an electric generator to regenerate electric energy by the kinetic energy of the hybrid vehicle which is transmitted from the drive wheels through the transmission to the electric motor. The regenerated electric energy is stored in an electric energy storage unit such as a battery used as a power supply for the electric motor.
The hybrid vehicle may be started by only the output power from the electric motor as disclosed in Japanese laid-open patent publication No. 5-38956, for example, or by the output power from both the engine and the electric motor while the engine is operating in a highest-efficiency torque range as disclosed in Japanese laid-open patent publication No. 8-317506, for example.
The output power applied to the drive wheels needs to be relatively large for smoothly starting the hybrid vehicle. Therefore, when the hybrid vehicle is started by only the output power from the electric motor, if the amount of electric energy stored in the electric energy storage unit for energizing the electric motor is relatively small, then the electric motor fails to generate a sufficient output power. As a result, the hybrid vehicle may not be started smoothly, or the electric energy stored in the electric energy storage unit may be rapidly consumed, resulting in an undue deterioration of the electric energy storage unit.
When the hybrid vehicle is started by the output power from both the engine and the electric motor while the engine is operating in a highest-efficiency torque range, inasmuch as the output power from the engine while operating in the highest-efficiency torque range is generally small, the hybrid vehicle has to rely highly upon the output power from the electric motor in order to be started smoothly. As a consequence, if the amount of electric energy stored in the electric energy storage unit for energizing the electric motor is relatively small, then the electric motor also fails to generate a sufficient output power. Thus, the hybrid vehicle may not be started smoothly, or the electric energy stored in the electric energy storage unit may be rapidly consumed, resulting in an undue deterioration of the electric energy storage unit.
The above drawbacks may be overcome by starting the hybrid vehicle with only the output power from the engine at all times. However, using only the output power from the engine to start the hybrid vehicle substantially increases the amount of fuel consumed by the engine.